In human hearing, hair cells in the cochlea respond to sound waves and produce corresponding auditory nerve impulses. These nerve impulses are then conducted to the brain and perceived as sound.
Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Conductive hearing loss typically occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, from damage to the ossicles. Conductive hearing loss may often be helped by using conventional hearing aids that amplify sounds so that acoustic information can reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
Many people who are profoundly deaf, however, have sensorineural hearing loss. This type of hearing loss can arise from the absence or the destruction of the hair cells in the cochlea which then no longer transduce acoustic signals into auditory nerve impulses. Individuals with sensorineural hearing loss may be unable to derive any meaningful benefit from conventional hearing aid systems no matter how loud the acoustic stimulus is. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural deafness, cochlear implant systems or cochlear prostheses have been developed that can bypass the hair cells located in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. This leads to the perception of sound in the brain and provides at least partial restoration of hearing function. Most of these cochlear prosthesis systems treat sensorineural deficit by stimulating the ganglion cells in the cochlea directly using an implanted electrode or lead that has an electrode array. Thus, a cochlear prosthesis operates by directly stimulating the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity to the connected auditory nerve cells.
Prior to stimulating the nerve cells, the electronic circuitry and the electrode array of the cochlear prosthesis separate acoustic signals into a number of parallel channels of information, each representing a narrow band of frequencies within the perceived audio spectrum. Ideally, each channel of information should be conveyed selectively to a subset of auditory nerve cells that normally transmit information about that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from the highest frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex.
A cochlear implant system typically comprises both an external unit that receives and processes ambient sound waves and a cochlear implant that receives data from the external unit and uses that data to directly stimulate the auditory nerve. A common configuration for a cochlear implant system thus involves internal components that are surgically implanted into the patient and external components that provide power and electrical signals representing environmental sound to the internal components. These external components typically include a Behind-the-Ear (BTE) processor worn on the ear or a body worn processor. These processors contain a microphone, batteries, and signal circuitry that processes the electrical signals generated by the microphone. The processors are connected to a headpiece by a cable. The headpiece receives the electrical signals through the cable and transmits them to the internal components.
In some cochlear implant systems, the cable or cables connecting the external components together can present some issues. For example, the cable may have to be routed through clothing or accommodated during hair styling. The cable may be snagged, pulled on, or tangled, causing the headpiece to fall off. Additionally, cables are considered unattractive by many patients and are susceptible to failure due to bending.
An inductive link is commonly used to transmit telemetry signals from the external unit to the implanted cochlear stimulator. To this end, the external unit often includes an inductive coil that produces a telemetry signal by generating an electro-magnetic field that is picked up by a receiver on the implanted cochlear stimulator. The inductive coil may be housed in an external headpiece that is positioned on a patient's head to transmit the telemetry signal through the patient's scalp to the implanted receiver. The external unit often includes a retention magnet for securing the headpiece to the patient's head so that the induction coil is properly positioned adjacent to the implanted receiver.
In a conventional cochlear implant system, electronic circuitry included within the external unit is not placed in relative close proximity to the induction coil and the retention magnet due to losses and interference caused by magnetic flux associated with the induction coil and the retention magnet. Hence, the electronic circuitry is typically housed within a behind-the-ear unit, for example, while the induction coil and the retention magnet are housed separately within a headpiece. Such a configuration is undesirable for many cochlear implant patients.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.